A known opening and closing device for a movable member (opening and closing body) is disclosed in Japanese Patent Laid-Open Publication No. 2000-335245. The known opening and closing device disclosed in Japanese Patent Laid-Open Publication No. 2000-335245 is applied to a backdoor provided on a rear portion of a vehicle as the opening and closing device of the movable member.
The opening and closing device disclosed in Japanese Patent Laid-Open Publication No. 2000-335245 includes a drive mechanism provided on a vehicle body side and an operation transmission mechanism for connecting the drive mechanism and the backdoor. The backdoor is opened and closed by transmitting the drive force of the drive mechanism to the backdoor via the operation transmission mechanism. The output of an electric motor included in the drive mechanism is transmitted to the operation transmission mechanism by the gear connection.
The opening and closing of the backdoor is generally assisted by a damper stay. The damper stay corresponds to a gas piston sealed with high pressure gas. The damper stay generates a resultant force in the closing direction added with the weight of the backdoor per se during the first half of the operation for opening the backdoor to prevent the sudden door opening. On the other hand, the damper stay assists to open the door by generating the resultant force in the opening direction added with the weight of the backdoor per se during the last half of the operation for opening the backdoor.
FIGS. 9a, 9b show the dynamic transmission between a drive gear (i.e., a gear on a motor side) 91 and a driven gear (i.e., a gear on a door side) 92 of the drive mechanism. FIG. 9a shows the first half of the door opening operation. FIG. 9b shows the last half of the door opening operation. For explanatory purpose, distances between adjacent gear teeth 91a of the drive gear 91 and distances between adjacent gear teeth 92a of the driven gear 92 are exaggerated. As shown in FIG. 9a, a force is applied to the driven gear 92 tending to urge the driven gear 92 in the counter direction to the rotational direction of the drive gear 91 during the first half of the door opening operation of the backdoor because the backdoor affecting the driven gear 92 applies a force in the closing direction during the first half of the door opening operation. Thus, the driven gear 92 follows the rotation of the drive gear 91 pushing the gear teeth 92a of the driven gear 92 with the gear teeth 91a thereof. Accordingly, the backdoor is moved in the opening direction via the driven gear 92.
On the other hand, as shown in FIG. 9b, during the last half of the door opening operation, when the backdoor is moved to a position exceeding a position balancing the force of the damper stay and the weight of the backdoor per se, the backdoor applies a force in the opening direction to urge the driven gear 92 in the same rotational direction as the drive gear 91. Because of the fluctuation in the rotational direction of the force applied to the driven gear 92 by virtue of the weight of the backdoor at the damper stay, the driven gear 92 moves within a backlash range relative to the drive gear 91 to suddenly move the backdoor. This deteriorates the smooth swinging movement of the door opening operation.
A need thus exists for an opening and closing device of an movable member for performing a relatively smooth opening and closing operation by restraining undesirable sudden swinging movement of the movable member.